Recently, regulations are being tight as to a requirement of reducing an amount of exhaust particulates (i.e., Particulate Matter) exhausted from an internal combustion engine. The regulations are tough especially in Europe, such that a quantity of the exhaust particulates is restricted in addition to a weight of the exhaust particulates. The same toughening of the regulations is expected even in Japan from now on. To respond to the toughening of the regulations, it is considered to collect the exhaust particulates by disposing a particulate filter (i.e., GPF: Gasoline Particle Filter) in an exhaust passage in addition to restricting a generation of the exhaust particulates by controlling air/fuel ratio in the internal combustion engine. Regarding vehicles mounting a diesel engine, it is already common to dispose the particulate filter in the exhaust passage, and the particulate filter has a great effect.
However, when the particulate filter is broken in some cases and a particulate collection performance of the particulate filter deteriorates, the quantity of the exhaust particulates passing through the particulate filter and emitted to an outside the vehicle may increase. The particulate filter may be broken in a case that a part of the particulate filter is broken when the exhaust particulates collected by the particulate filter are burnt and a temperature of the part of the particulate filter increases excessively. Then, it is considered to dispose a particulate detection device so as to detect a failure of the particulate filter promptly. The particulate detection device uses a particulate sensor that is located downstream of the particulate filter in the exhaust passage and detects an amount of the exhaust particulates.
Patent Literature 1 discloses a particulate detection device that has a particulate sensor having electrodes. The particulate sensor has an electrical insulating part having a plate shape, and the electrodes are arranged on a surface of the electrical insulating part to be distanced from each other. The exhaust particulates are a conductor including carbon as a base. Accordingly, electrical resistance between the electrodes adjacent to each other decreases when the exhaust particulates are attached on the surface of the electrical insulation part and accumulated in a portion between the electrodes. That is, an accumulated amount of the exhaust particulates on the electrical insulation part and the electrical resistance between the electrodes correlate with each other. Regarding an internal combustion engine of Patent Literature 1, the particulate detection device detects the amount of the exhaust particulates accumulated on the surface of the electrical insulation part, i.e., an amount of the exhaust particulates in the exhaust passage, based on the electrical resistance between the electrodes. The electrical resistance is actually a current value that is detected while voltage is applied between the electrodes.
According to the particulate sensor having the above-described configuration, the electrical resistance between the electrodes is decreased as the exhaust particulates are accumulated, and then the current detected increases and is saturated eventually. That is, the current stops increasing eventually, i.e., the electrical resistance stops decreasing, even if the accumulated amount of the exhaust particulates continues increasing. Therefore, it is necessary to regenerate the particulate sensor in order to continue detecting the amount of the exhaust particulates. The regeneration is performed by removing the exhaust particulates in a manner that the exhaust particulates are burnt by heating the electrical insulation part routinely. The process to regenerate the particulate sensor will be referred to as “regeneration treatment”. The particulate sensor disclosed in Patent Literature 1 has a heater heating the electrical insulation part and performs the regeneration treatment using the heater.